1. Field of the Invention
This invention relates to a two-component binder for use in the production of exterior-grade fiberboard and other wood products, to the composition of one of such components and its process of manufacture, to a process of producing such binder and to fiberboards impregnated with the binder.
2. Discussion of the Prior Art
Lignocellulosic products such as wood fiberboard, particle board and plywood materials are usually treated during their assembly with phenolic or similar type resins to impart waterproof characteristics to the product. For example, in the fiberboard industry, phenolic resins are used to produce hardboard by a hot-press method wherein the wood fibers are impregnated with the resin, an impregnated mat is formed therefrom and the impregnated mat is compressed to a desired thickness between pressure platens at elevated temperatures and elevated pressures for a considerable time. In some operations, partially polymerized urea-formaldehyde resins are blended with melamine-formaldehyde resins to produce the desired waterproof bond. In order to be rated as exterior-grade materials, such products must pass the accelerated aging tests of ASTM Test Method D-1037. Due to the necessity of utilizing a hot-press method for curing the phenolic resin, which involves a slow cure of the resin, the rates of production are considered slow for commercial operations. Additionally, some of the above-mentioned resins are excessively tacky which presents problems in providing a homogeneous impregnation of the wood fiber or particles and creates pluggage problems in conveying the impregnated material prior to its formation into a board. Consequently, there currently is no medium density (e.g., 37 to 50 pounds per cubic foot) fiberboard made with a phenolic binder. Moreover, phenolic resin based systems must be cured by the hot platen press method as opposed to the faster radio frequency cure methods.
Medium density fiberboard for exterior siding has been previously produced using a urea-melamine-formaldehyde resin as the binder. Such system required the handling of four components in a reactor and the resultant resin had a very short shelf life (e.g., on the order of about 1 day). In such a system, an aqueous concentrate of urea and formaldehyde was mixed with melamine, urea and water, heated for a time sufficient to dissolve the melamine and blended with a curing catalyst. Although such binder exhibited little tack and was useful for binding fiberboards which were cured by a radio frequency equipped press, its shelf life necessitated in situ formation in the fiberboard plant involving the use of four components in a carefully controlled process.
Amino resin binders useful for fiberboard and plywood are also described in U.S. Pat. No. 3,629,176 to Shriver, wherein melamine, and optionally urea and water, are added at a temperature no higher than 100.degree. F. to a urea-formaldehyde reaction product to obtain a slurry which can be employed to bond fiberboard and the like but which has a relatively short stable life. Thus, like the system described in the preceding paragraph, such a system must be formulated in the fiberboard plant since the solids of the slurry would separate out after a relatively short period of time so as to preclude shipment of such product in slurry form.
U.S. Pat. No. 3,458,464 to Shriver et al. discloses aqueous urea-formaldehyde-melamine concentrates in which melamine and urea are added to an aqueous formaldehyde solution which is heated and then concentrated after cooling. Although it is suggested in such patent that urea and melamine may be dissolved in the liquid to obtain a reaction mixture useful in producing adhesives, molding powders and fabric-treating resins, the concentrate itself contains a very large amount of fomaldehyde in order to render the concentrate stable and a very small amount of melamine. Consequently, there is an odor problem associated with the large amount of formaldehyde present in such concentrate. The system as envisioned in the Shriver et al. patent is at least a three-component system wherein urea and melamine plus the urea-formaldehyde-melamine concentrate are employed. Moreover, it has been found that the product of Shriver et al. becomes unstable (i.e., becomes solid) after about three days' storage.
A two-component curable amino resin is described in U.S. Pat. No. 3,891,590 to Sundie et al. However, it has been found that such resin is not useful for exterior grade fiberboard meeting the requirements of ASTM D-1037.
U.S. Pat. No. 2,085,492 to Ellis (1937) discloses carrying out the reaction of urea and acid-containing methanolic formaldehyde in the presence of additional methanol solvent or other volatile diluent miscible with water in order to lessen the thermal effect of the reaction and to obtain a soluble product; the solvent is recovered after the reaction is completed.
U.S. Pat. No. 3,826,770 to Christensen et al. (1974) discloses urea-formaldehyde aqueous concentrates useful in the manufacture of particle board and the like, which contain an ionized inorganic salt as a stabilizer.
Australian Patent Application 60,444 to Montecatini (1960) discloses a melamine-formaldehyde solution in a hydroalcoholic medium, such as methanol, and containing an aromatic sulfamide as stabilizing agent.
British Plastics, August 1953 (pages 306-308) discloses aqueous melamine-formaldehyde syrups which are stabilized against gellation and precipitation by the incorporation of an alcohol and borax.
It would desirable if a two-component binder system for exterior grade fiberboard and other wood products and the like were provided, which system was sufficiently stable so that it could be shipped separately to fiberboard manufacturing facilities and stored for commercially practical times.